The present invention relates to the field of ultrasonic imaging as used for example in medical diagnostic scanning. More particularly, the invention relates to an improved ultrasonic imaging system that is capable of accurately displaying the velocity distribution of movement of blood flow within a living organism.
Ultrasound imaging systems for medical diagnostics scan a subject with ultrasound beams in a field of sector or rectangular pattern and provide a two-dimensional display of the scanned field. The imaging system includes a transducer that emits and focuses the ultrasound beams along certain linear directions or scan lines. The emitted beams reflect back, in the form of an echo of acoustic scattering to the transducer along the same scan lines. Subsequently, the received beams are converted to electrical signals, which are then processed to generate an image on a two-dimensional display representative of a planar cross-section through the subject. The resolution of the images depends on how well the beams are locally focused along the scan lines, as well as the number of scan lines per sector scanned.
Modern ultrasound imaging systems may scan a subject to provide a tissue image (or B-mode image) and a blood flow image (or color flow image) of the scanned sector field. Generally, B-mode imaging produces a two-dimensional gray scale image of the tissue of the subject. The imaging system generates a single frame of a B-mode image by scanning multiple beams in a given sector or rectangular pattern. The ultrasound imaging systems sequentially emit and receive a plurality of beams (or transmissions) through the subject. Specifically, the beams are swept through the sector at small lateral increments to generate one frame of a B-mode image. Each frame includes as many as 60 to 400 laterally spaced beams.
In addition, color flow imaging has been developed to display blood flow as a two-dimensional color image. A B-mode image and a color flow image are superimposed to provide a composite image of the blood flow through the tissue or organ of the subject being scanned. The principle of detecting blood flow may be either the Doppler technique or a time-shift technique. For the Doppler technique, an autocorrelation technique has been implemented in most commercial ultrasound systems. This autocorrelation technique in principle can obtain a velocity or a phase shift between two consecutive complex Doppler signals by calculating the complex product between the two after wall filtering. However, in reality, averaging of several pairs is necessary to enhance blood flow sensitivity by increasing signal to noise ratio in the signal from the human body.
The prior art technique of color flow imaging is diagrammatically shown in FIG. 1. The upper panel 10 illustrates multiple transmit/receive ultrasound beams 12 that create a color flow image output line 14, shown in the lower panel 16. The prior art ultrasound system transmits and receives a plurality of packets or groups 18 of ultrasound beams 12 at spaced lateral positions 20-24 along the imaging field of the subject being scanned. Each packet 18 comprises approximately between 6-16 ultrasound beams. The packets of beams are transmitted at an interval, or pulse repetition time (PRT) at each position. For instance, as a single frame of the imaging field is scanned, the ultrasound imagine(, system transmits and receives a first packet of ultrasound beams (8 beams) at a first position 20. The second packet of beams are scanned at a second position 21 laterally-spaced from the first position. Further packets 18 of beams 12 are emitted and received in the lateral direction until the entire field is scanned. Five (5) lateral positions 20-24 are illustrated to depict a sample of packets 18 of beams for scamming the field.
As a result, five (5) color flow output lines 14 are calculated and displayed in the lower panel 16 for each respective packet 18 of ultrasound beams 12. Each beam 12 is representative of received and complex Doppler signals of varying depth or range in the body. For example, solid circles 26 in the upper panel 10 depict a subpacket 28 of received and digitized signals representative of the body structure of a subject at a predetermined depth in the body or distance from the transducer. The solid circle 30 in the lower panel 16 shows a flow velocity output calculated from the subpacket 28 of the received and digitized signals (solid circles) 26 shown in the upper panel 10. One skilled in the art will recognize that each ultrasound beam comprises many (up to several hundred) received and digitized signals 26, and each color flow output line 14 comprises a corresponding number of calculated velocities 30, although not specifically shown in the FIG. 1.
In order to capture motions of tissues or blood flow at reasonably fast frame rate, the number of color flow output lines 14 is limited and far less than the number of B-mode beams. As shown hereinbefore, each color flow image output line 14 represents eight (8) transmitted color flow beams 12 and therefore, the B-mode image comprises approximately eight times as many received output signals as the color flow image for an equal number of scan B-mode and color flow scan lines for each frame. Consequently, the color flow output lines 14 density is much less than that of B-mode output beams, resulting in lower spatial resolution than that of B-mode images.
This invention offers advantages and alternatives over the prior art by providing an ultrasound imaging system that provides for high density scanning (transmit/receive) of color flow signals. The ultrasound scan sequence of each transmitted and received color flow ultrasound beam is laterally spaced through the imaging field to provide a high density of color flow lines. The scanning technique of the present invention enables a large number of color flow lines to be acquired while keeping a high frame rate. The color line density is comparable to that of B-mode images. A method of high-speed calculation, or a high-speed autocorrelation is also provided to process the greater number of signals generated by the present scanning technique and therefore, maintain a high frame rate. A time sharing scanning technique between color flow and B-mode images is also provided in order to more accurately synchronize blood flow and tissue (B-mode) images together. Furthermore, color flow beams may also used for B-mode imaging to thereby increase the frame rate.
In accordance with an embodiment of the present invention, a method for generating color flow images for an imaging system includes scanning sequentially a predetermined number of color flow beams through an imaging field wherein the color flow beams are laterally spaced through the imaging field. The color flow beams define a first packet of color flow beams. The first packet of color flow beams is processed to generate a first color flow output signal. An additional color flow beam, which is laterally spaced from a last color flow beam of the first packet, is scanned. The color flow beams of the first packet and the additional color flow beam, excluding a first color flow beam of the first packet, define a second packet. The second packet of color flow beams is processed to generate a second color flow output signal. This process continues for many color flow beams. A color flow image is generated from the first and second color flow signals as well as many other signals.
Preferably, the processing of the second packet of color flow beams includes using the results of the autocorrelation of complex signals representative of each color flow beam of the first packet at a predetermined depth. A complex product of a complex signal of the last color flow beam of the first packet and a complex signal of the additional color flow beam at the predetermined depth is determined as a new complex product. A complex product of a complex signal of the first color flow beam of the first packet and a complex signal of a second color flow beam of the first packet at the predetermined depth is determined as an old complex product. The autocorrelation, the new complex product and negative of the old complex product are summed to determine a complex signal or an autocorrelation representative of the color flow output signal in the second color output line at the predetermined depth.
In accordance with another embodiment of the present invention, a method for scanning color flow beams and B-mode beams of an imaging system for one frame of an imaging field includes scanning color flow beams through a first portion of the imaging field. B-mode beams are scanned through the first portion of the imaging field. Additional color flow beams are scanned through a second portion of the imaging field, which is adjacent to the first portion after scanning color flow beams and B-mode beams through the first portion of the imaging field. B-mode beams are scanned through the second portion of the imaging field after scanning color flow beams and B-mode beams through the first portion of the imaging field.